Backside Photon Emission Microscopy (PEM) is commonly used for circuit diagnostics and analyses of VLSI (Very-Large-Scale Integration) devices (chips). The premise of PEM is that individual logic gates within a VLSI circuit emit “Hot Carrier” (HC) photons when switching states. These photons are generally in the Infrared (IR) part of the spectrum, and since Silicon is transparent at these wavelengths, it is possible to observe the circuit (Device Under Test, or DUT) in action through its back side (the substrate side, opposite to the metal layer side).
Electron-hole recombination is a mechanism that dominates in forward-biased p-n junctions; this can arise in bipolar or BiCMOS circuits, latched-up CMOS, some types of gate or power-supply shorts, and some poly stringer conditions, to name a few. The emission is relatively narrow spectrally, and is centered near 1150 nm. Forward-biased p-n diodes emit this light even in the absence of a strong electric field. The emission comes from bipolar recombination, not from hot carriers. Thus this signal occurs at low voltage. In general, it is not possible to have a p-n junction forward-biased with more than 1-2 V due to the extremely high current densities that would be obtained. So the case of high voltage is not particularly important; although, if high forward bias voltages could be achieved, the spectrum would be nearly identical to the low-voltage case.
Cameras (detector arrays) sensitive across the IR range are readily available, with frequency response shown in FIG. 1. Commonly, MCT cameras (Mercury Cadmium Telluride, HgCdTe) are used for this purpose since their response is uniform across a wide slice of the spectrum extending up to LWIR (about 18 um). Other types of detectors such as MOS CCD, Indium Antimonide (InSb) or Indium-Gallium-Arsenide (InGaAs) are also commonly used.
The spectral characteristic of the emissions from the semiconductor gates depends on many factors, such as excitation voltage, defect type and fabrication technology. A significant part of the emission lies in wavelengths beyond the traditional threshold of 1.55 um (commonly observed by InGaAs cameras operating at liquid Nitrogen temperature).
For common VLSI devices, however, the HC emissions are very faint, and since the amount of noise originating in thermal emissions (which follows the black body radiation spectral distribution) increases with wavelength, it interferes more with observations at these longer wavelengths.
In each band of the spectrum, therefore, are present both HC emissions from the DUT, which constitute the signal, and thermally-originated emissions, from both the DUT and the optics of the microscope, which contribute to the noise. Having a large signal-to-noise ratio (SNR) is important for achieving good observations.
The faint HC emissions also cause the exposure times to be as long as hundreds of seconds, which complicates the observation. One way to shorten such exposure times is to increase the SNR.
Some existing designs limit the range of observation to 1.5 um (which coincides with the sensitivity of InGaAs cameras) and since thermal noise is rather weak at these wavelengths, such a system works great for devices operating voltage above 800 mV.
For such wavelengths, passive designs are used to mitigate thermal noise that originates outside of the nominal optical path of the microscope, but they cannot completely eliminate it, nor can they eliminate thermal noise that originates within the optical path. In prior art systems, a relay lens is placed between the objective and the detector, and a cold aperture is placed between the relay lens and the detector in a position corresponding to the image location of the aperture of the objective, as imaged by the relay lens. This arrangement minimizes stray thermal radiation that is emitted by the body of the camera. For further information the reader is referred to, e.g., U.S. Pat. No. 6,825,978,